This invention relates to protector circuits for protecting communication equipments for analog or digital transmission systems from lightning surges applied to signal lines.
Protector circuit standards for digital transmission systems with a transmission speed of 200 kb/sec. are as follows.
Limit voltage (i.e., breakdown voltage of communication equipment) V.sub.R is no higher than 320 V.
Battery feed voltage (i.e., voltage supplied from a telephone exchange to a subscriber's communication equipment) V.sub.Bf is 30 V (between core line and ground).
Non-operation voltage (i.e., the sum of the 50/60 Hz induced voltage on commercial power line, crosstalk of telephone bell ringer voltage and battery feed voltage, at which non-operation voltage the protector circuit should not be operated) V.sub.A is 100 V. In a protector circuit in an analog transmission system, the non-operation voltage is the ringer voltage of 120 V. And the protector circuit is designed such that it will not be operated at this ringer voltage of 120 V.
Electrostatic capacitance C is no higher than 250 pF. (Transmission loss standards specify that the signal line impedance with respect to ground should be 4 k.OMEGA. or above for the maximum frequency component of the transmitted signal.)
Surge current withstanding capability (i.e., current caused through the protection circuit by application of a lightning surge waveform [10/1,000] .mu.sec rising to a peak voltage in 10 .mu.sec. and exponentially falling to one half the peak voltage in 1,000 .mu.sec) I.sub.p is greater than 58A.
FIG. 1 shows a prior art protector circuit for switchboard analog circuits. The circuit comprises a series connection of resistor 14, fuse 15 and resistor 16 and another series connection of resistor 17, fuse 18 and resistor 19, these series connections being inserted in pair signal lines (i.e., subscriber's line) 12 and 13 connected to subscriber's communication equipment 11. Resistors 14 and 17 are grounded at the ends thereof opposite from switchboard 11 through a three-electrode gas-filled protector (discharge tube) 21. Resistors 16 and 19 are grounded at the both ends thereof through zinc oxide varistors 22 to 25.
When a lightning surge voltage is applied to signal line 12, discharge is caused in the three-electrode gas-filled protector 21 between signal line 12 and ground. Due to the resultant discharge light or ions, the other side of the discharge tube 21 between signal line 13 and ground is triggered to discharge. The voltage on signal lines 12 and 13 consequently becomes the arc voltage of discharge tube 21, which is 20 to 50 V, thus protecting equipment 11 from the lightning surge voltage.
The discharge trigger voltage and discharge delay time of the discharge tube, however, vary greatly, i.e., 300 to 800 V and 0.3 to 3.0 .mu.sec., depending on the input surge voltage rising time. In addition, the response time of the three-electrode gas-filled protector is large. Before the discharge of discharge tube 21 is initiated, the lightning surge voltage appears across varistors 22 to 25, so that communication equipment 11 is protected by the "on"-state voltage across varistors 22 to 25 until the sum of the "on"-state voltage across varistors 22 and 24 and the voltage drop across resistors 14 and 17 is sufficiently increased to start discharge of three electrode gas-filled protector 21.
In the protector circuit shown in FIG. 1, it is necessary to make up for fluctuations of the discharge characteristics of three-electrode gas-filled protector 21. Also, it is necessary to solve problems due to the delay of discharge. To ensure necessary surge protection characteristics, varistors, resistors, etc. are used. The circuit, therefore, comprises a large number of components and inevitably has a large size. In addition, the varistors should withstand high surge voltage, leading to an extra electrostatic capacitance.
FIG. 2 shows another prior art protector circuit which is designed for digital transmission systems. This circuit uses, in lieu of varistors, series connections of a plurality of bilateral voltage limiting elements 26, e.g., bilateral zener diodes (avalanche diodes) are used. In order to reduce capacitance, the series connection of bilateral voltage limiting elements 26 is fabricated as a lamination. The lamination has a large number of layers and requires a complicated manufacturing process.
Meanwhile, the varistor has a low voltage non-linearity index. Therefore, even if a multi-stage surge protection circuit is employed (e.g., three-stage in the case of FIG. 1), the communication equipment 11 should be designed to have a high breakdown voltage to cover variation in limiting voltage of the varistor.
While the protector circuits shown in FIGS. 1 and 2 are actually used, there has been proposed a protector circuit, which does not use any lightning tube but uses thyristors, as shown in FIG. 3. This circuit is disclosed in U.S. Pat. No. 4,322,767 (filed on Feb. 11, 1980) entitled "Bidirectional Solid-State Protector Circuitry Using Gated Diode Switches". In this protector circuit, upon application of a positive lightning surge voltage to point A, the voltage at point 1200 is clamped by Zener diode Z2 to a fixed voltage. At this time, zener diode Z3 is turned on so that the lightning surge voltage is applied to point 1800. The voltage at point 1800 thus exceeds the voltage at point 1200 to cause current to the gate of unidirectional thyristor GDSA, thus turning on the thyristor GDSA. When the applied lightning surge voltage is reduced, zener diode Z3 is turned off to turn off thyristor GDSA. Zener diodes Z1 and Z4 and thyristor GDSB are provided for negative lightning surge voltages.
This protector circuit has a voltage detector, the output of which is fed to the gate of the thyristor to turn on the thyristor. The voltage detector includes a large number of components and has a complicated construction. In addition, zener diodes Z3 and Z4 can not be constructed as a single element. FIG. 3 shows only a portion of the circuit that is connected to signal line 12 shown in FIG. 1, that is, it is necessary to connect the same circuit to signal line 13. Further, zener diodes Z3 and Z4 are necessary for controlling the thyristor gate, and their breakdown voltage can not be determined in relation to the battery feed voltage on the signal line.
FIG. 4 shows another prior art protector circuit disclosed in U.S. Pat. No. 4,282,555 (filed on Aug. 13, 1979) entitled "Overvoltage Protection Means for Protecting Low Power Semiconductor Components". When the voltage on signal line A exceeds the sum of forward "on" voltage of diode D1 and break-over voltage V.sub.B0 of unidirectional thyristor T3 as a result of application of a positive lightning surge voltage, diode D1 and thyristor T3 are turned on. When the lightning surge voltage is reduced so that the surge current becomes lower than holding current I.sub.H of unidirectional thyristor T3, thyristor T3 is turned off. When a negative lightning surge voltage is applied to signal line A, diode D3 and thyristor T1 are turned on.
Where the current versus voltage characteristic of power supplied to the communication equipment via signal lines is as shown by line 27 in FIG. 5, holding current I.sub.H of thyristors T1 and T3 should be above maximum current I.sub.m. In order to attain a large holding current I.sub.H, conventional surge protector circuits employ thyristors of an emitter short-circuited structure, resulting in increased chip areas therefor. In addition, break-over voltage V.sub.B0 of the thyristor is set to be higher than non-operation voltage V.sub.A lest the thyristor should turn on at non-operation voltage V.sub.A. Therefore, the current versus voltage characteristic of the thyristor has been set as shown by curve 28. The thyristor thus momentarily consumes a high power of V.sub.B0 .times.I.sub.H (shown shaded in the Figure) when it is turned on. To increase the surge withstanding capability, therefore, the thyristor has been designed to have a large chip area. In this manner, the conventional surge protector circuit must use thyristors of increased chip areas, thus increasing capacitance to deteriorate the signal transmission characteristics of the signal lines. This method, therefore, is not suited for high speed transmission systems. Further, heating of the thyristor due to lightning surge current therein reduces its holding current as shown by dashed line 29. Until the thyristor temperature is lowered to cause the holding current to exceed the supply current, the thyristor is held in ON-state, while the supply current flows therethrough, disabling the communication equipment. That is, communication remains interrupted until the holding current becomes higher than the supply current as the thyristor temperature falls again. For example, when the holding current of the thyristor which is 150 mA at normal temperature is reduced to 0 due to a rise of the thyristor peak temperature to 250.degree. to 300.degree. C. caused by a lightning surge, the thyristor temperature should become lower than 100.degree. C. to regain a holding current in excess of the supply current of 120 mA. This takes a time period of at least 10 msec. During this period of at least 10 msec., the communication is interrupted. To obtain quick thyristor temperature fall, it is necessary to use a double heatsink diode structure of silver, a good heat conductor, leading to a high price.
In the protector circuits shown in FIGS. 3 and 4, an independently operable protector circuit is connected to each signal line. Therefore, when a lightning surge is applied with phase differences to a plurality of signal lines leading from a communication equipment, or when there are fluctuations of the timing of start of operation of the protection circuits to the individual signal lines, a large transverse mode voltage is liable to be produced between signal lines through the communication equipment connected therebetween. Such a transverse mode voltage will cause rupture of the communication equipment.
French Pat. No. 2,498,387 discloses a further protector circuit. In this circuit, a voltage detector is used to detect a surge voltage, and the detector output is used to simultaneously control the gates of bidirectional thyristors connected between the pair signal lines and ground, respectively, to simultaneously turn on the two thyristors. The transverse mode voltage noted above, therefore, is not generated in this case. However, the voltage detector for detecting the surge voltage is complicated. In addition, a delay is involved in the surge detection. Therefore, a grounded bidirectional voltage limiting element is connected between each signal line and ground to provide protection against surges until the thyristor is turned on.